The present invention is directed to ellagic acid and certain derivations which are useful as anti-inflammatory and antiallergic agents by virtue of their ability to inhibit phospholipase A.sub.2.
It is now well-established that arachidonic acid (AA) is metabolized in mammals by two distinct pathways. The metabolism of arachidonic acid by cyclooxygenase enzymes results in the production of prostaglandins and thromboxanes. The physiological activity of the prostaglandins has already been amply elucidated in recent years. It is now known that prostaglandins arise from the endoperoxides PGG.sub.2 and PGH.sub.2 by the cyclooxygenase pathway of arachidonic acid metabolism. These endoperoxides are also the precursors of the thromboxanes (Tx) A.sub.2 and B.sub.2. TxA.sub.2 is a vasoconstrictor which stimulates platelet aggregation. In the normal situation, the vasoconstrictive and platelet aggregating properties of the thromboxanes are balanced by another product arising from the endoperoxides in the cyclooxygenase pathway, prostacyclin (PGI.sub.2), which is a vasodilator with platelet aggregation inhibitory activity. In the event prostacyclin synthesis is impaired and/or platelet activation is enhanced, then thrombosis and vasoconstriction is favored. The role of prostanoids in haemostasis and thrombosis are reviewed by R. J. Gryglewski, CRC Crit. Rev. Biochem., 7, 291 (1980) and J. B. Smith, Am. J. Pathol., 99, 743 (1980). Cyclooxygenase metabolites are known to participate directly in the inflammatory response [see Higgs et al., Annals of Clinical Research, 16, 287-299 (1984)]. This is through their vasodepressor activities, participation in pain and fever, augmentation of peptide mediator vascular permeability and edema forming properties. Finally, various aspects of cell mediated immunity are influenced by cyclooxygenase products.
The other pathway of AA metabolism involves lipoxygenase enzymes and results in the production of a number of oxidative products called leukotrienes. The latter are designated by the LT nomenclature system, and the most significant products of the lipoxygenase metabolic pathway are the leukotrienes B.sub.4, C.sub.4 and D.sub.4. The substance denominated slow-reacting substance of anaphylaxis (SRS-A) has been shown to consist of a mixture of leukotrienes, with LTC.sub.4 and LTD.sub.4 as the primary products and having varying amounts of other leukotriene metabolites [see Back et al., J. Immun., 215, 115-118 (1980); Biochem. Biophys. Res. Commun., 93, 1121-1126 (1980)].
The significance of these leukotrienes is that a great deal of evidence has been accumulated showing that leukotrienes participate in inflammatory reactions, exhibit chemotactic activities, stimulate lysosomal enzyme release and act as important factors in the immediate hypersensitivity reaction. It has been shown that LTC.sub.4 and LTD.sub.4 are potent bronchoconstrictors of the human bronchi [see Dahlen et al., Nature, 288, 484-486 (1980) and Piper, Int. Arch. Appl. Immunol., 76, suppl. 1, 43 (1985)] which stimulates the release of mucus from airways in vitro [Marom et al., Am. Rev. Resp. Dis., 126, 449 (1982)], are potent vasodilators in skin [see Bisgaard et al., Prostaglandins, 23, 797 (1982)], and produce a wheal and flare response [Camp et al., Br. J. Pharmacol., 80, 497 (1983)]. The nonpeptide leukotriene, LTB.sub.4, is a powerful chemotactic factor for leukocytes [see A. W. Ford-Hutchinson, J. Roy. Soc. Med., 74, 831-833 (1981)], which stimulates cell accumulation and affects vascular smooth muscle [see Bray, Br. Med. Bull., 39, 249 (1983)]. The activity of leukotrienes as mediators of inflammation and hypersensitivity is extensively reviewed in Bailey and Casey, Ann. Reports Med. Chem., 19, 87 (1986).
Phospholipase A.sub.2 (PLA.sub.2) is the critical rate limiting enzyme in the arachidonic acid (AA) cascade since it is responsible for the hydrolysis of esterified AA from the C-2 position of membrane phospholipids. This reaction generates two products: (1) free AA which is then available for subsequent metabolism by either the cyclooxygenase or lipoxygenase enzymes, and (2) lysophospholipid. When alkylarachidonoyl-glycerophosphatidylcholine is acted upon by the PLA.sub.2 the generation of platelet activating factor (PAF) is initiated; PAF is pro-inflammatory in its own right [see Wedmore et al., Br. J. Pharmacol., 74, 916-917 (1981)]. In this regard it may be noted that the anti-inflammatory steroids are thought to inhibit eicosanoid synthesis by inducing the synthesis of a PLA.sub.2 inhibitory protein denominated macrocortin, lipomodulin or lipocortin [see Flower et al., Nature, London, 278, 456 (1979) and Hirata et al., Proc. Natn. Acad. Sci. U.S.A., 77, 2533 (1980)].
As the initial step leading to subsequent conversion of AA to the various eicosanoids by the cyclooxygenase and lipoxygenase pathways, the PLA.sub.2 -mediated release of AA from membrane phospholipids is a critical event in attempting to deal with the various physiological manifestations which are based on the activity of the eicosanoids and/or PAF. Thus, while PLA.sub.2 has been shown to be required for platelet aggregation [Pickett et al., Biochem. J., 160, 405 (1976)], cardiac contraction and excitation [Geisler et al., Pharm. Res. Commun., 9, 117 (1977)], as well as prostaglandin synthesis [Vogt, Adv. Prostagl. Thromb. Res., 3, 89 (1978)], the inhibition of PLA.sub.2 is indicated in the therapeutic treatment of both PAF induced or cyclooxygenase and/or lipoxygenase pathway product-mediated physiological conditions. Thus, PLA.sub.2 inhibitors are a rational approach to the prevention, removal or amelioration of such conditions as allergy, anaphylaxis, asthma and inflammation.
Ellagic acid (4, 4', 5, 5', 6, 6'-hexahydrodiphenic acid 2, 6, 2', 6'-dilactone), I, is a naturally occurring substance which has been obtained from guava, rasberries, walnuts, and cloves (see, U.S. Pat. No. 3,576,007). ##STR4## The isolation and synthesis of ellagic acid and its ether and ester derivatives are well known in the prior art (see, Beilstein Vol. 19, p. 285, 2nd Ed. and Vol. 19, p. 3164, 3/4 Ed.)
Ellagic acid, its derivatives, and salts thereof have been shown to be useful as hemostatic agents [see, Derwent Publications 82-8884OE, 66-40347F, 66-33545F and U.S. Pat. No. 3,576,007) by virtue of their ability to activate Hageman factor (Ratnoff et al., J. Lab. Clin. Med. 63, 359 (1964)]. In addition, 3,3'dimethyl ellagic acid has been shown to possess antitumor activity (Derwent Publications 78-88538A).